Optical information carrier having buffer spaces for maintaining its flat shape

ABSTRACT

Optical information carrier comprising an optically transparent layer and a reflective layer adjacent to each other whereby the interface comprises an optically readable information structure containing alternate areas with such different properties, so that a scanning light beam is modulated in correspondence with the information present in said structure. Between the surface of optically transparent layer and the surface of the reflective layer, one or more gas-filled interconnected buffer spaces are provided at positions described over the interface between the layers, and an excavation is provided at the edge of the carrier.

CROSS REFERENCE TO RELATED APPLICATION

This is a division of application Ser. No. 253,396 filed Oct. 3, 1988now abandoned which is a continuation of Ser. No. 852,039, filed on Feb.20, 1986, which is now abandoned.

BACKGROUND OF THE INVENTION

The invention relates to an optical information carrier comprising anoptically transparent layer and a reflective layer adjacent to eachother, whereby the interface comprises an optically readable informationstructure containing alternate areas with such different properties,that a scanning light beam is modulated in correspondence with theinformation present in said structure.

Information carriers of this type in a broad sense of the meaning areknown for various purposes and in various embodiments. Examples of suchinformation carriers are for instance optical lattice grids, hologramsand audio discs or video discs (compact disc).

A general problem with these known optical information carrierstructures is the fact that in general gases and vapours, especiallywater vapour, are able to diffuse into the optical transparent layer butnot through the reflective layer. Depending upon the environmentalconditions, especially during a reduction of the atmospheric pressureand/or the water vapour concentration, water vapour and/or other gaseswill diffuse to the outside. At that side of the optical transparentlayer directed to the atmosphere there are no obstructions for such adiffusion. However, at the side where the reflective layer is realized,said diffusion causes problems. The reflective layer in most casesconsists of metal which does not transmit gases and vapours. In case thevapour pressure at the interface between the optical transparent layerand the reflective layer increases under the influence of environmentalconditions, then it is not inconceivable that the adherence between bothlayers is locally not able to withstand this increase in pressure andwill collapse. The result thereof is the development of larger orsmaller bubbles or blisters at the interface between both layers. Suchblisters form a serious hindrance for the error-free reading of theinformation present in the carrier structure. Taking into account thesometimes very small dimensions of the information structure, thedimensions on a video disc are in the order of some μm, the developmentof even the smallest gas or vapour bubbles in said interface must beprevented.

The presence of a diffusion-tight reflective layer can also result in adistortion of the information carrier as a whole, because the vapourprofile of a layer will in general be symmetrical in the directiontowards the side edges, assuming that the same atmospheric pressureprevails at both sides and that a free exit and respectively a freepenetration of gas and vapour is possible. However, if one side of thelayer is closed in a vapourtight and gastight way, such as with theoptical information carrier of the above-mentioned type, then in generalan asymmetrical dampness or vapour profile will be developed in theoptical information carrier. Such asymmetrical vapour profile causesmechanical tensions in the carrier, which may result in a mechanicaldistortion of the carrier as a whole, which phenomenon could beindicated by the term "warping". With an optical information carrier, asfor instance a compact disc, this warping phenomenon will soon lead to asituation in which the information read-out apparatus is no longer ableto read the stored information.

SUMMARY OF INVENTION

The object of the invention is now to embody and provide the aboveindicated optical information carrier such that, regardless of changesin the environmental conditions the forming of blisters and thedevelopment of mechanical tensions, blisters and distortions, etc. inthe carrier is prevented.

In agreement with said object, the optical information carrier of theabove-mentioned type is characterized in that one or more gas-filledbuffer spaces are provided on the interface between the surface of thetransparent layer and the surface of the reflecting layer of the carrierat various positions.

The effect of the distributed buffer spaces in the interface betweenboth layers is that also at the side of the reflective layer, vapour ordampness can diffuse out of the optically transparent layer into thebuffer spaces, so that the vapour pressure at the interface between theoptically transparent layer and the reflective layer is not or hardlyincreased. The forming of bubbles or blisters at the interface betweenboth layers is therewith effectively prevented. Furthermore, the bufferspaces are preferably created such that the vapour profile, through thewhole optically transparent layer as a result of the presence of thebuffer spaces, remains symmerical or at least to a large extentsymmetrical, such that the creation of mechanical tensions which couldlead to warping of the whole carrier structure is also prevented.

The buffer spaces are created by making cavities or excavations eitherin the reflective layer or in the optically transparent layer.Preferably the buffer spaces are formed by creating one or more cavitiesor excavations in the optically transparent layer at the side of thereflective layer in the same process step in which the opticalinformation carrier structure is also formed.

Furthermore the shape, dimensions or embodiments of the buffer spacesare preferably selected such that said buffer spaces can bedistinguished optically during the read-out of the optical informationcarrier.

In case, for instance with a circular information carrier, the bufferspaces are directed radially in relation to the information structure ofthe carrier, optically recognizable buffer spaces can be used to dividethe carrier into sectors comprising different information.

In another embodiment, the buffer spaces are created between theinformation tracks on the carrier in a spiral shape or a circular shape.In these cases, the buffer space can be used to control the readinglight beam in relation to the information carrier along the informationtracks.

These examples indicate that the buffer space or buffer spaces can formpart of the optical information structure on the carrier. In anotherexample, the information carrying structure comprises opticallyrecognizable higher and lower parts forming piles on a flat opticallytransparent substrate. In that case, the space between the piles canform part of the buffer space or may form said buffer space itself.

It is clearly illustrated in these examples that the buffer spaces canbe mutually connected. It has appeared that preferably the buffer spaceshave to be mutually connected such that the pressure equalization at theinterface between the optical transparent layer and the reflective layeris at a maximum.

A preferred embodiment of the optical information carrier according tothe invention is characterized in that, in the optical transparent layeror in the reflective layer along the edge or edges of the informationstructure, an excavation is provided with a relatively high depth inrelation to the buffer spaces, which excavation communicates with thebuffer space(s). Depending upon the dimensions of the excavation, asignificant enlargement of the buffer zone can be created in this way.

A further improvement can be realized under circumstances in case therelatively deep excavation(s) are in connection with the atmosphere. Inthat case a general pressure equalization is obtained at both sides ofthe transparent layer.

In case the excavation extends along the edge of the carrier, then it ispreferred that near the edge of said carrier inside the excavation asupporting element (16) is installed whose thickness is equal to thedepth of the excavation. The result thereof is a mechanicalstrengthening of the carrier, especially a stiffening of the edge of thecarrier. When viewed in a direction parallel to the interface betweenthe optical transparent layer and the reflective layer, the outerdimension of the supporting element is preferably equal to the outerdimension of the carrier, and the inner dimension is larger than theinner dimension of the excavation.

To realize the best possible vapour pressure equalization, it ispreferred that said supporting element (16) be made of a vapourtransmissible material, such as poly(methyl methacrylate), polyvinylalcohol, cellophane, cellulose acetate or a material in foamedcondition, such as for instance polyurethane, polyvinyl chloride,poly(methyl metacrylate). The supporting element (16) can also be madeof metal and contains one or more passages through which the buffer zonecommunicates with the atmosphere.

DESCRIPTION OF INVENTION

Further preferred embodiments, details and further advantages of theinvention will be described in the following description with referenceto embodiments illustrated in the attached drawings.

FIG. 1 illustrates a cross section through a part of an opticalinformation carrier structure according to the invention.

FIG. 2 illustrates a cross section through a part of another embodimentof an optical information carrier according to the invention and locatedalong the edge of said carrier.

FIG. 3 illustrates a cross section near the edge of an audio disc orvideo disc containing buffer zones and an excavation according to theinvention.

The optical information carrier 1 in FIG. 1 is assembled from anoptically transparent layer 2 which, as shown in the Figure at the underside, comprises an optically readable information structure comprising anumber of information tracks 7a, 7b, 7c, 7d. In a further way notindicated in detail, each track comprises alternative areas with suchdifferent properties that a scanning light beam (not shown) will bemodulated in correspondence with the information present in thestructure. From the prior art various ways are known to embody suchoptically readable information tracks and to provide said tracks inpractice. The optically transparent layer 2 is closed at the under sideby the reflective layer 3, for instance embodied as a metal layer whichis polished on least at one side.

Between the information tracks 7a, 7b, 7c, 7d buffer spaces 4 arepresent, realized by means of multiple cavities or grooves made in theoptically transparent layer 2. In case the part of the informationcarrier illustrated in FIG. 1 belongs to an information carrier with aspirally shaped information track part of which is indicated by thesections 7a, 7b, 7c, 7d, then it will be clear that in fact all thebuffer spaces 4 in FIG. 1 are communicating with each other and that infact all said spaces form a spirally-shaped groove positioned inbetweenthe information tracks 7. Depending upon the build-up of the informationstructure, connecting passages may be present within the tracks 7a, 7b,by means of which passages the cavities 4 are directly mutuallycommunicating.

The optically transparent layer 2 is in general made of a plasticmaterial in which water vapour or gas can diffuse. The reflective layer3 is in general made of a metal in which no water vapour or gas candiffuse. In case the buffer spaces or cavities 4 are omitted, then thetransparent layer 2 would be closed gas-tight and water-tight at oneside by the reflective layer 3, with the result that at the underside nogas or water vapour can diffuse out of the transparent layer 2.Depending upon the environmental conditions, the result thereof will bea relatively high vapour pressure or gas pressure at the interfacebetween the layers 2 and 3 which might cause a local separation betweenboth layers and the forming of so-called blisters, small gas or vapourbubbles. As will be clear, the reading of the information tracks will beseriously disturbed by such blisters or even made impossible.

Due to the presence of the cavities 4, water vapour or gas may diffuseout of the transparent layer 2 at the under side. Said gas or watervapour reaches the hollow spaces 4, but the pressure increase createdthereby is almost negligible and will certainly not lead to the formingof blisters.

FIG. 2 illustrates a further embodiment of an information carrier 11according to the invention comprising an optically transparent layer 12and a reflective layer 13. The layer 12 carries also in this case theschematically indicated information tracks 17a, 17b, . . . , eachcomprising alternating areas. The buffer spaces are in this case formedby means of grooves 14 in the reflective layer 13.

Along the edge of the carrier in FIG. 2, a relatively deep excavation orgroove 15 is provided in the reflective layer 13, which excavation isthrough transversal grooves in a not illustrated way communicating withthe ends of a spirally formed groove 14 embodying the buffer space.Because of said deep excavation 15 the volume of the buffer zone as awhole, comprising the mutually communicating spaces 14, is significantlyincreased and the function of said buffer zone is enhanced thereby.

In this embodiment furthermore a supporting element ring 16 is installedalong the edge of the disc, which ring 16 forms a structural connectionbetween the transparent layer 12 and the reflective metal layer 13. Thering 16 is preferably made of a plastic material, such as poly(methylmethacrylate), polyvinyl alcohol, cellophane or cellulose acetate, or ismade of a foamed plastic such as polyurethane, polyvinyl chloride, orpoly(methyl methacrylate). In all cases, a direct vapour connection or adiffusion connection with the atmosphere is created such that an evenmore optimal functioning of the buffer zone created by the spaces 15 and14 will be guaranteed. Instead thereof or as an addition, passages canbe made through the ring 16 to assure a pressure equalization betweenthe buffer zone and the atmosphere.

The result of a direct connection or a diffusion connection with theatmosphere is furthermore that the vapour pressure, especially the watervapour pressure, at both sides of the transparent layer 12 will be equalor almost equal. We have found that the diffusion of water vapourthrough the mutually communicating buffer zones is taking place so fast,that the vapour pressure in the whole buffer space is almost equal tothe vapour pressure in the atmosphere. The development of anasymmetrical vapour pressure profile with the resulting mechanicaltensions, which easily could lead to warping of the whole informationcarrier making said information carrier useless, is therewith prevented.

Finally, FIG. 3 illustrates a cross section taken through an embodimentof an audio or video information carrier disc similar to the embodimentin FIG. 1, which disc contains a relatively deep excavation. The discstructure 21 illustrated in FIG. 3 comprises an optically transparentlayer 22, the underside of carries the information tracks 27a, 27b, 27c.Between said tracks the buffer spaces or zones 24 are created by forminggrooves in the transparent layer 22 between the information tracks. Thetracks 27a, 27b, 27c may consist of series of piles in the same way asis illustrated in FIG. 2, but can also be realized in another known way.

Along the edge of the information carrier structure 21 in the embodimentshown in FIG. 3, a relatively deep excavation 25 is provided in theoptically transparent layer 22 communicating with the mutuallyinterconnected buffer spaces 24. The deep excavation 25 results in asignificant volume increase of the total buffer zone consisting of themutually interconnected spaces 24, and thereby results in an enhancedfunctioning of said buffer zones 24.

Furthermore also in this FIG. 3 embodiment, a ring element 26 isinstalled along the information edge of the carrier disc 21, which ring26 forms a connection between the carrier optically transparent layer 22and the reflective metal layer 23. As shown in FIG. 3, the outer radialdimension of the supporting ring element 26 of disc 21 is equal to theouter radial dimension of the carrier disc, and the inner radialdimension of the supporting ring element 26 is greater than the innerradial dimension of the excavation 25. Preferably again said ring 26 ismade of a plastic material such as poly(methyl methacrylate), poly vinylalcohol, cellophane or cellulose acetate, or is made of a foamed plasticsuch as polyurethane, polyvinyl chloride or poly(methyl methacrylate).In all cases, a direct communication vapour connection or a diffusionconnection with the atmosphere is created for vapour pressureequalization, such that the optimum functioning of the buffer zonecomprising the buffer spaces 25 and 24 is guaranteed. Instead of or asan addition, the ring 26 may contain passages to guarantee a pressureequalization between the buffer zones 24 and 25 and the atmosphere.

With reference to preventing the forming of blisters in opticalinformation carriers, the invention proves its advantages with allmaterials in which gas or vapour can diffuse. In relation to the warpingeffect, the invention has special advantages with carriers made ofplexiglass [poly(methy methacrylate)] and, although to a lesser extent,carriers made of polycarbonate material.

I claim:
 1. An optical information carrier comprising an optically transparent layer and a reflective layer provided adjacent to each other, wherein the interface between said layers comprises an optically readable information structure containing alternate areas with such different properties that a scanning light beam is modulated in correspondence with the information present in said structure, said information carrier being characterized in that, between the surface of the optically transparent layer and the adjacent surface of the reflective layer, multiple gas-filled buffer spaces are provided at positions distributed over the interface between said layers, said buffer spaces being mutually connected so as to provide pressure and vapor equalization between the buffer spaces, the buffer spaces being formed by multiple cavities made in the optically transparent layer adjacent the reflective layer and extended into areas intermediate between tracks of the information structure, wherein in the optically transparent layer along the edge or edges of the information structure, an excavation (25) is provided having a relatively greater depth in relation to the buffer spaces, said excavation communicating with the buffer spaces.
 2. An optical information carrier according to claim 1, wherein the shape and dimensions of the buffer spaces are selected such that the buffer spaces can be optically distinguished from the information structure.
 3. An optical information carrier according to claim 2, wherein the optically readable information structure comprises higher and lower parts in the optically transparent layer, whereby the space or spaces present between the reflective layer and the lower parts of the information structure form at least part of the buffer space.
 4. An optical information carrier according to claim 3, wherein the higher parts are determined by the upper surfaces of piles extending from the lower surface of the information carrier, the upper surfaces of said piles being parallel to the lower surface of the information carrier.
 5. An optical information carrier according to claim 1, wherein the relatively deep excavation (25) communicates with the atmosphere.
 6. An optical information carrier according to claim 1, wherein a supporting element (26) is provided near the edge of the carrier excavation, the thickness of said supporting element (26) being substantially equal to the depth of the excavation.
 7. An optical information carrier according to claim 6, wherein when viewed in a direction parallel to the interface between the optically transparent layer (22) and the reflective layer (23), the outer radial dimension of the supporting element (26) is equal to the outer radial dimension of the carrier (21), and the inner radial dimension of the supporting element (26) is greater than the inner radial dimension of the excavation (25).
 8. An optical information carrier according to claim 6, wherein the supporting element (26) is made of a material which is transmissible for gases and/or vapors including water vapor.
 9. An optical information carrier according to claim 8, wherein the supporting element is made of a material selected from the group consisting of poly(methyl methacrylate), polyvinyl alcohol, cellophane and cellulose acetate.
 10. An optical information carrier according to claim 8, wherein the supporting element is made of a foamed plastic material selected from the group consisting of foamed polyurethane, foamed polyvinyl chloride and foamed poly(methyl methacrylate).
 11. An optical information carrier according to claim 6, wherein the supporting element is made of a material which is transmissible for gas or vapor, and the supporting element (16) contains at least one passage through which the buffer zones communicate with the atmosphere. 